1. Field of the Invention
This invention involves a process for cracking, desulfurizing, and demetalating a hydrocarbon fraction.
2. Description of the Prior Art
As a result of the increasing demand for light hydrocarbon fractions, there is much current interest in more efficient methods for converting the heavier hydrocarbon fractions and products of refining into lighter materials. The conventional methods of accomplishing this, such as catalytic cracking, coking, thermal cracking and the like, always result in the production of more highly refractory materials.
It is known that such heavier hydrocarbon fractions and products and such refractory materials can be converted to lighter materials by hydrocracking. Hydrocracking processes are most commonly employed on liquefied coals or heavy residual or distillate oils for the production of substantial yields of low boiling saturated products and to some extent of intermediates which are utilizable as domestic fuels, and still heavier cuts which find uses as lubricants. These destructive hydrogenation processes or hydrocracking processes may be operated on a strictly thermal basis or in the presence of a catalyst.
However, the application of the hydrocracking technique has in the past been fairly limited because of several interrelated problems. Conversion of heavy petroleum products and hydrocarbon fractions to more useful products by the hydrocracking technique is complicated by the presence of certain contaminants in heavier hydrocarbon fractions and refinery products. Petroleum crude oils and the heavier hydrocarbon fractions and/or distillates obtained therefrom, particularly heavy vacuum gas oils, oil extracted from tar sands, and topped or reduced crudes, contain nitrogenous, sulfurous, and organo-metallic compounds in exceedingly large quantities. The presence of sulfur- and nitrogen-containing and organo-metallic compounds in crude oils and various refined petroleum products and hydrocarbon fractions has long been considered undesirable.
For example, because of the disagreeable odor, corrosive characteristics and combustion products (particularly sulfur dioxide) of sulfur-containing compounds, sulfur removal has been of constant concern to the petroleum refiner. Further, the heavier hydrocarbons are largely subjected to hydrocarbon conversion processes in which the conversion catalysts are, as a rule, highly susceptible to poisoning by sulfur compounds. This has led in the past to the selection of low-sulfur crudes whenever possible. With the necessity of utilizing heavy, high sulfur hydrocarbon fractions in the future, economical desulfurization processes are essential. This need is further emphasized by recent and proposed legislation which seeks to limit sulfur contents of industrial, domestic, and motor fuels.
Generally, sulfur appears in feedstocks in one of the following forms: mercaptans, hydrogen sulfides, sulfides, disulfides, and as part of complex ring compounds. The mercaptans and hydrogen sulfides are more reactive and are generally found in the lower boiling fractions, for example, gasoline, naphtha, kerosene, and light gas oil fractions. There are several well-known processes for sulfur removal from such lower boiling fractions. However, sulfur removal from higher boiling fractions has been a more difficult problem. Here, sulfur is present for the most part in less reactive forms as sulfides, disulfides, and as part of complex ring compounds of which thiophene is a prototype. Such sulfur compounds are not susceptible to the conventional chemical treatments found satisfactory for the removal of mercaptans and hydrogen sulfide and are particularly difficult to remove from heavy hydrocarbon materials.
Nitrogen is undesirable because it effectively poisons various catalytic composites which may be employed in the conversion of heavy hydrocarbon fractions. In particular, nitrogen-containing compounds are effective in suppressing hydrocracking. Moreover, nitrogenous compounds are objectionable because combustion of fuels containing these impurities possibly contributes to the release of nitrogen oxides which are noxious and corrosive and present a serious problem with respect to pollution of the atmosphere. Consequently, removal of the nitrogenous contaminants is most important and makes practical and economically attractive the treatment of contaminated stocks.
However, in order to remove the sulfur or nitrogen or to convert the heavy residue into lighter more valuable products, the crude oil or heavy hydrocarbon fraction is ordinarily subjected to a hydrocatalytic treatment. This is conventionally done by contacting the oil or hydrocarbon fraction with hydrogen at an elevated temperature and pressure and in the presence of a catalyst. Unfortunately, unlike distillate stocks which are substantially free from asphaltenes and metals, the presence of asphaltenes and metal-containing compounds in the heavy hydrocarbon fractions leads to a relatively rapid reduction in the activity of the catalyst to below a practical level. The presence of these materials in the charge stock results in the deposition of metal-containing coke on the catalyst particles, which prevents the charge from coming in contact with the catalyst and thereby, in effect, reduces the catalytic activity. Eventually, the on-stream period must be interrupted, and the catalyst must be regenerated or replaced with fresh catalyst.
Particularly objectionable is the presence of iron in the form of soluble organometallic compounds, such as is present frequently to a relatively high parts-per-million level in Western United States crude oils and residuum fractions. Even when the concentration of iron porphyrin complexes and other iron organometallic complexes is relatively small -- that is, on the order of parts per million -- their presence causes serious difficulties in the refining and utilization of heavy hydrocarbon fractions. The presence of an appreciable quantity of the organometallic iron compounds in feedstocks undergoing catalytic cracking causes rapid deterioration of the cracking catalysts and changes the selectivity of the cracking catalysts in the direction of more of the charge stock being converted to coke. Also, the presence of an appreciable quantity of the organo-iron compounds in feedstocks undergoing hydroconversion (such as hydrotreating or hydrocracking) causes harmful effects in the hydroconversion processes, such as deactivation of the hydroconversion catalyst and, in many instances, plugging or increasing of the pressure drop in fixed bed hydroconversion reactors due to the deposition of iron compounds in the interstices between catalyst particles in the fixed bed of catalyst.
Additionally metallic contaminants such as nickel- and vanadium-containing compounds are found as innate contaminants in practically all crude oils associated with the high Conradson carbon asphaltic and/or asphaltenic portion of the crude. When the crude oil is topped to remove the light fractions boiling above about 450.degree.-650.degree. F., the metals are concentrated in the residual bottoms. If the residuum is then further treated, such metals adversely affect catalysts. When the oil is used as a fuel, the metals also cause poor fuel oil performance in industrial furnaces by corroding the metal surfaces of the furnace.
There have been numerous references to processes for hydrogenating, cracking, desulfurizing, denitrifying, demetalating, and generally upgrading hydrocarbon fractions by processes involving water. For example Gatsis, U.S. Pat. No. 3,453,206 (1969) discloses a multistage process for hydrorefining heavy hydrocarbon fractions for the purpose of eliminating and/or reducing the concentration of sulfurous, nitrogenous, organo-metallic, and asphaltenic contaminants therefrom. The nitrogenous and sulfurous contaminants are converted to ammonia and hydrogen sulfide. The stages comprise pretreating the hydrocarbon fraction, in the absence of a catalyst, with a mixture of water and externally supplied hydrogen at a temperature above the critical temperature of water and a pressure of at least 1,000 pounds per square inch gauge and then reacting the liquid product from the pretreatment stage with externally supplied hydrogen at hydrorefining conditions and in the presence of a catalytic composite. The catalytic composite comprises a metallic component composited with a refractory inorganic oxide carrier material of either synthetic or natural origin, which carrier material has a medium-to-high surface area and a well-developed pore structure. The metallic component can be vanadium, niobium, tantalum, molybdenum, tungsten, chromium, iron, cobalt, nickel, platinum, palladium, iridium, osmium, rhodium, ruthenium, and mixtures thereof.
Gatsis, U.S. Pat. No. 3,501,396 (1970) discloses a process for desulfurizing and denitrifying oil which comprises mixing the oil with water at a temperature above the critical temperature of water up to about 800.degree. F. and at a pressure in the range of from about 1,000 to about 2,500 pounds per square inch gauge and reacting the resulting mixture with externally supplied hydrogen in contact with a catalytic composite. The catalytic composite can be characterized as a dual function catalyst comprising a metallic component such as iridium, osmium, rhodium, ruthenium and mixtures thereof and an acidic carrier component having cracking activity. An essential feature of this method is the catalyst being acidic in nature. Ammonia and hydrogen sulfide are produced in the conversion of nitrogenous and sulfurous compounds, respectively.
Pritchford et al., U.S. Pat. No. 3,586,621 (1971 ) discloses a method for converting heavy hydrocarbon oils, residual hydrocarbon fractions, and solid carbonaceous materials to more useful gaseous and liquid products by contacting the material to be converted with a nickel spinel catalyst promoted with a barium salt of an organic acid in the presence of steam. A temperature in the range of from 600.degree. F. to about 1,000.degree. F. and a pressure in the range of from 200 to 3000 pounds per square inch gauge are employed.
Pritchford, U.S. Pat. No. 3,676,331 (1972) discloses a method for upgrading hydrocarbons and thereby producing materials of low molecular weight and of reduced sulfur content and carbon residue by introducing water and a catalyst system containing at least two components into the hydrocarbon fraction. The water can be the natural water content of the hydrocarbon fraction or can be added to the hydrocarbon fraction from an external source. The water-to-hydrocarbon fraction volume ratio is preferably in the range of from about 0.1 to about 5. At least the first of the components of the catalyst system promotes the generation of hydrogen by reaction of water in the water gas shift reaction and at least the second of the components of the catalyst system promotes reaction between the hydrogen generated and the constituents of the hydrocarbon fraction. Suitable materials for use as the first component of the catalyst system are the carboxylic acid salts of barium, calcium, strontium, and magnesium. Suitable materials for use as the second component of the catalyst system are the carboxylic acid salts of nickel, cobalt, and iron. The process is carried out at a reaction temperature in the range of from about 750.degree. to about 850.degree. F. and at a pressure of from about 300 to about 4000 pounds per square inch gauge in order to maintain a principal portion of the crude oil in the liquid state.
Wilson et al., U.S. Pat. No. 3,733,259 (1973) discloses a process for removing metals, asphaltenes, and sulfur from a heavy hydrocarbon oil. The process comprises dispersing the oil with water, maintaining this dispersion at a temperature between 750.degree. and 850.degree. F. and at a pressure between atmospheric and 100 pounds per square inch gauge, cooling the dispersion after at least one-half hour to form a stable water-asphaltene emulsion, separating the emulsion from the treated oil, adding hydrogen, and contacting the resulting treated oil with a hydrogenation catalyst at a temperature between 500.degree. and 900.degree. F. and at a pressure between about 300 and 3,000 pounds per square inch gauge.
It has also been announced that the semi-government Japan Atomic Energy Research Institute, working with the Chisso Engineering Corporation, has developed what is called a "simple, low-cost, hot-water, oil desulfurization process" said to have "sufficient commercial applicability to compete with the hydrogenation process." The process itself consists of passing oil through a pressurized boiling water tank in which water is heated up to approximately 250.degree. C., under a pressure of about 100 atmospheres. Sulfides in oil are then separated when the water temperature is reduced to less than 100.degree. C.
Thus far, no one has disclosed the method of this invention for upgrading hydrocarbon fractions, which permits operation at lower than conventional temperatures, without an external source of hydrogen, and without preparation or pretreatment of the hydrocarbon fraction, such as, desalting or demetalation.